The present invention relates generally to a hydraulic parking brake controller of a motor vehicle and, more specifically, to a hydraulic parking brake controller used in construction machines and industrial vehicles in order to control, in response to an operation unit, the operation of a hydraulic parking brake provided at the vehicle output shaft.
Parking brake controllers have been installed in construction machines and industrial vehicles. In those vehicles a drum type or a disc type parking brake is provided on the output shaft of a vehicle transmission, which brake is operated by an operator manually in order to brake the output shaft.
Nowadays, other brake controllers have been developed in which a normally-engaged type parking brake on the transmission output shaft is released by hydraulic pressure. A conventional brake controller of this type is shown in FIG. 5 and illustrates a parking brake 2 provided around an output shaft 1 of a transmission. The parking brake 2 comprises a case 2a secured to a housing of the motor vehicle, a plurality of brake discs 3, a piston 4 and a spring 5 for urging the piston 4 axially against brake discs 3. Some of the brake discs 3 are engaged with the case 2a so as to be slidable in the axial direction yet non-rotatable. Other of the brake discs 3, interlaminated with the former discs, are non-rotatably engaged with the output shaft 1 and are slidable in the axial direction. The piston 4 is driven leftward in FIG. 5 by the spring 5 to compress the brake discs 3, thereby braking the output shaft 1.
A directional control valve 7 is connected to an oil chamber 6 defined by the case 2a of the parking brake 2. The directional control valve 7 comprises a case 7a, a piston valve 8 and a solenoid 9 which extends or retracts the piston valve 8 in order to control the engagement and the release of the parking brake 2. The directional control valve 7 is connected to a hydraulic power source 10 and a reservoir 14. The hydraulic power source supplies hydraulic fluid to the directional control valve 7. The solenoid 9 provides on-off control in response to signals from an operation unit 11 installed at the driver's seat, which unit 11 is connected to an electric source 13 through a main switch 12.
When the contact of the operation unit 11 is switched over to its actuation terminal, the solenoid 9 switches off, moving the piston valve 8 to the position shown by the solid lines in FIG. 5. As a result, the hydraulic fluid in the oil chamber 6 of the parking brake 2 drains into the reservoir 14 through the oil chamber 7a of the directional control valve 7 whereupon the brake discs 3 are compressed under the urging force of the spring 5 between the brake housing and the piston 4. Thus the output shaft 1 is braked.
When the contact of the operation unit 11 is switched over to its release terminal, the solenoid 9 switches on, moving the piston valve 8 to the position shown by the dashed lines in FIG. 5. Thereupon, hydraulic fluid is supplied to the oil chamber 6 of the parking brake 2 from the hydraulic power source 10, so that the piston 4 moves rightward in FIG. 5 against the urging force of the spring 5, whereby the output shaft 1 can rotate freely.
If the circuit to the solenoid 9 in the aforedescribed parking brake controller, should become broken or short, the solenoid 9 switches off, actuating the parking brake 2. Thus in the event of electrical malfunction or it the hydraulic system is otherwise out of order, the parking brake will nonetheless be engaged.
However, in the conventional brake controllers, the hydraulic fluid in the oil chamber 6 of the parking brake 2 is drained very suddenly when the solenoid 9 is deactivated on account of electrical malfunction such as wire breaks, such that the brake is engaged too suddenly. The strong jolting of such sudden braking can be very dangerous to machine operators.